"Much work remains to be done and we must not relax our guard," said Yukiya Amano, director general of the International Atomic Energy Agency, at the start of an IAEA-hosted conference aimed at enhancing international standards to prevent any repeat of Japan's reactor meltdowns.

"The accident may have faded from international headlines but it is essential that all of us — member states, the IAEA and other key stakeholders— maintain our sense of urgency," the veteran Japanese diplomat said.

He was addressing delegates at the extraordinary meeting of the 75-nation Convention on Nuclear Safety (CNS), which was negotiated after the 1986 nuclear disaster at the Chernobyl nuclear plant near Kiev, which sent radioactive dust across Ukraine, Russia, Belarus and western Europe.

Li Ganjie of China's National Nuclear Safety Administration, president of the week-long meeting, said nuclear safety "must know no boundaries", even though this may raise costs. "Without nuclear safety there can be no nuclear power development."

Meltdowns at the Fukushima nuclear plant after the March 2011 earthquake and tsunami sent radiation spewing over large areas, forcing more than 160,0000 people to flee. In the following months, all Japan's remaining reactors were shut for safety checks. Two reactors resumed operation last month.

Japan's Fukushima plant. Reuters

The worst such accident since Chernobyl also cast a question mark over the future of nuclear energy elsewhere in the world. In Europe, Germany, Switzerland and Belgium decided to move away from nuclear to increase their reliance on renewable energy.

The IAEA has said it believes, however, that global use of nuclear energy could increase by as much as 100 percent by 2030 on the back of growth in Asia, including in China and India.

Amano said "significant progress has been made in several key areas" after the adoption of an IAEA safety action plan last year, including assessments of safety vulnerabilities of nuclear power plants and improvements in emergency preparedness.

"I know you will make good use of this opportunity to consider further measures to strengthen nuclear safety throughout the world," Amano told the CNS states, which include all countries with nuclear power plants except Iran.

Some nations criticised the IAEA plan, approved six months after the Fukushima accident, for recommending voluntary steps instead of mandatory measures.

A senior Japanese official, Shinichi Kuroki, later briefed the conference about conditions at Fukushima, saying the reactors were stable but that decommissioning posed challenges.

Fukushima "is not in a position where we expect any further accidents," said Kuroki, deputy director general of Japan's nuclear and industry safety agency, NISA.

The reactors are "being cooled off in a stable manner" and there is "containment of further radioactivity emitted", he added.